Descriptions

The primary Mg/Ca ratio of foraminiferal shells is a potentially valuable paleoproxy for sea surface
temperature (SST) reconstructions. However, the reliable extraction of this ratio from sedimentary calcite
assumes that we can overcome artifacts related to foraminiferal ecology and partial dissolution, as well as
contamination by secondary calcite and clay. The standard batch method for Mg/Ca analysis involves cracking,
sonicating, and rinsing the tests to remove clay, followed by chemical cleaning, and finally acid-digestion and
single-point measurement. This laborious procedure often results in substantial loss of sample (typically 30–
60%). We find that even the earliest steps of this procedure can fractionate Mg from Ca, thus biasing the result
toward a more variable and often anomalously low Mg/Ca ratio. Moreover, the more rigorous the cleaning, the
more calcite is lost, and the more likely it becomes that any residual clay that has not been removed by physical
cleaning will increase the ratio. These potentially significant sources of error can be overcome with a flow-through
(FT) sequential leaching method that makes time- and labor-intensive pretreatments unnecessary. When
combined with time-resolved analysis (FT-TRA) flow-through, performed with a gradually increasing and
highly regulated acid strength, produces continuous records of Mg, Sr, Al, and Ca concentrations in the leachate
sorted by dissolution susceptibility of the reacting material. Flow-through separates secondary calcite from less
susceptible biogenic calcite and clay, and further resolves the biogenic component into primary and more
resistant fractions. FT-TRA reliably separates secondary calcite (which is not representative of original life
habitats) from the more resistant biogenic calcite (the desired signal) and clay (a contaminant of high Mg/Ca,
which also contains Al), and further resolves the biogenic component into primary and more resistant fractions
that may reflect habitat or other changes during ontogeny. We find that the most susceptible fraction of biogenic
calcite in surface dwelling foraminifera gives the most accurate value for SST and therefore best represents
primary calcite. Sequential dissolution curves can be used to correct the primary Mg/Ca ratio for clay, if
necessary. However, the temporal separation of calcite from clay in FT-TRA is so complete that this correction is
typically ≤2%, even in clay-rich sediments. Unlike hands-on batch methods, that are difficult to reproduce
exactly, flow-through lends itself to automation, providing precise replication of treatment for every sample.
Our automated flow-through system can process 22 samples, two system blanks, and 48 mixed standards in
<12 hours of unattended operation. FT-TRA thus represents a faster, cheaper, and better way to determine
Mg/Ca ratios in foraminiferal calcite.